This invention relates to the means for reduction of induced drag on the airlifting surfaces of aircraft, helicopters, and windmills. Further more, this idea provides for design of new flying-wing-concept aircraft with sufficient payload space, increased longitudinal stability and maneuverability, and reduced profile and wave drag at high subsonic, as well as supersonic speeds when compared to the existing flying-wing-concept aircraft.
Induced drag on the lifting surfaces, which is generated on the tips thereof due to pressure difference between their lower and upper surfaces, represents a significant problem especially with highly loaded lifting surfaces where the ratio between aircraft weight and wing surface area is high.
Extensive theoretical and practical research showed that induced drag on the tips of airlifting surfaces depended on the square of lift coefficient, as well as the shape and aspect ratio of the airlifting surfaces. The most favorable shape is the elliptical one. However, it is not used often due to its geometry definition and related manufacturing complexity. The increase of airlifting surface aspect ratio is limited due to resulting unfavorable structural loading and requires increase in structural weight, which further requires higher attack angles of the airlifting surfaces to produce extra lift necessary to carry additional weight, whereby resulting in induced drag increase.
Due to above reasons, for the past couple of decades, efforts to solve this problem have been concentrated to modify the tips of airlifting surfaces similar to birds with low-aspect-ratio wings. Efforts to solve the problem of induced drag on the tips of lifting surfaces have been done in three different ways:
a) By sweeping back the tip of airlifting surfaces with significant narrowing and tilt thereof upward or downward.
b) By creating a net of smaller airlifting surfaces at the tip of the main airlifting surface
c) By splitting the tips of airlifting surfaces to a number of smaller airlifting surfaces
The following patents relate to the ways of solving the problems of induced drag as reflected under a):
U.S. Pat. No. 5,634,613 discloses a highly swept leading edge of the tip of an airlifting surface, which is tilted downwardly and simultaneously significantly narrowed. Tilting the tip of the airlifting surface downward will not produce significant positive effects on reduction of induced drag since induced drag is caused mostly by low pressure being created on the upper
U.S. Pat. No. 4,477,042 discloses similarly a tip of an airlifting surface that is tilted, however upwardly, which is more favorable. All of these solutions just marginally reduce induced drag.
U.S. Pat. No. 4,205,810 discloses a winglet that is tilted upwards at 90-degree angle relatively to the wing chord plane. The winglet extends along three-quarters of the wing tip chord length starting from aft portion thereof The significant difference in the air pressure at the forward wing tip portion between the upper and under wing surface will initiate vortex formation, which will not be suppressed by the winglet since the forward wingtip portion is not bound by the winglet. The 90-degree angle between the upper surface of wing and the winglet will cause turbulence, which will cause additional interference drag.
U.S. Pat. No. 4,245,804 eliminates some of the shortcomings pertinent to U.S. Pat. No. 4,205,810 by adding a forward strake to the winglet, which extends along the forward wingtip portion. Also, the winglet is tilted relative to the vertical plane for an angle Ø with a transition area between the upper surface of wing and the winglet, thereby partially reducing both induced and interference drag.
U.S. Pat. No. 5,348,253 eliminates further some of the shortcomings of the U.S. Pat. No. 4,245,804 by having a transition section inserted between the main portion of wing and the winglet, which curves upwards and extends along the entire wing tip chord. The transition is used for increased lift production and additional reduction of both induced and interference drag.
The following patents relates to the ways of solving the problems of induced drag as reflected under b):
U.S. Pat. No. 4,671,473 discloses a number of winglets that are pivotally connected to the main wing tip and have adjustable sweep and attack angle. In addition to the problems arising from the complexity of mechanisms necessary to manipulate with winglets and difficulties related to the space available for the disposal of such mechanisms inside the thin wing tip portion of the main wing, a significant interference drag is generated between the main lifting portion and winglets, as well as among densely distributed winglets especially at higher flight speed when the attack angles of winglets are low and consequently the slots among them are narrow. This disclosure is practically inapplicable to the aircraft that fly at high speed.
Similarly, Patent No. CH 684588 A5 discloses a net of winglets being mutually connected that have ability to be extracted and retracted from and into the main wing respectively. Interference drag among winglets is eliminated when winglets are retracted into the main wing, but this solution is not satisfactory due to added weight and the complexity of the method for retraction and extraction of winglets.
U.S. Pat. No. DE 19706668 A1 discloses a net of winglets mutually connected to an aerodynamically profiled rod that is connected to the wing tip. Winglets are mutually dispersed in the vertical plane thereby reducing interference drag especially at the winglets tips. However, the problems of interference drag between the rod and the winglets, the rod and the wing, and the problem of rod drag itself still remained.
The following patents relate to the ways of solving the problems of induced drag as reflected under c):
U.S. Pat. Nos. 4,545,552 and 4,674,709 disclose tips of a lifting surface being split to forward and aft winglet sections that curve and extend in the opposite directions in the vertical plane where the forward and aft winglets are additionally provided with movable surfaces for pitch, yaw, and roll control. However, these patents do not define a method for split and formation of two separate airfoils from a single airfoil at the tip of lifting surface in order to avoid sudden transition and high interference drag.
Patent No. DE 19752369 A1 discloses a tip of a lifting surface that is split into curved forward and aft sections and then reconnected again forming a closed loop. This disclosure is favorable from the aspect of aerodynamic loading and rigidity of split tips, but it is complex from the manufacturing aspect. Also, it does not disclose how the tips of lifting surfaces are split in order to reduce the interference drag, which is also the case with U.S. Pat. No. 4,545,552 and 4,674,709.
U.S. Pat. No. 4,722,499 discloses a tip of a lifting surface being split into two winglets and tilted in vertical plane in the opposite directions by inserting an xe2x80x9caerodynamic boomxe2x80x9d between a wing and the winglets. This disclosure is similar to the one in Patent No. DE 19706668, but it has additional formers in the area where winglets are connected to the aerodynamic boom for the purpose of interference drag reduction. The winglet that is oriented downward does not give as positive drag reduction effects as it is the case with the winglet that is oriented upwards. Aerodynamic boom additionally increases weight and drag.
U.S. Pat. No. 3,270,988 of Sep. 6, 1966 treats highly loaded surfaces of wings with standard aspect ratio by suggesting to split wing tips in a number of smaller winglets that are lined up in the direction of airflow, differentially arched, and rigidly joined the wing""s main section, thereby achieving a wing tip configuration similar to the one of eagle during gliding, which significantly reduces induced drag relatively to the conventional wing with the same span and surface area. However, the inventor does not offer any vision or a concrete idea, or a suggestion as to how to perform such a wing division on real constructions in order to avoid turbulence and high interference drag in the division area, as well as flow separation on the wing""s upper surface with simultaneous preservation of wing tip aerodynamic efficiency.
Tendencies to eliminate huge parasite drag, which is created by the aircraft fuselage, led to the idea of xe2x80x9cflying wingxe2x80x9d where wings have been used for payload accommodation. However, this idea is facing two key problems in practical terms:
How to provide for sufficient height and volume for passenger and payload accommodation using airfoils of standard relative thickness
How to provide for sufficient longitudinal stability and maneuverability
Some of the early flying wing designs filed due to insufficient longitudinal stability, which resulted in crashes. This significantly slowed down further development of this idea. The first next practical attempt in this direction was B-2 aircraft. The B-2 design was characterized with thin airfoils adapted for a relatively high speed of flight. These airfoils are deformed and thickened in the cockpit area. Insufficient longitudinal stability was compensated for by programmable computerized systems to maintain longitudinal stability under different flight regimes, which certainly does not meet civil aviation regulation requirements.
The next significant project based on the flying wing idea, which is still in the design phase, is the Blended Wmg Body aircraft (BWB). It is designed for civil aviation (transport of up to 800 passengers) and has large dimensions (wing span of 87 m and length 49 m) with a maximum relative thickness of airfoils between 17 and 20 percent in the passenger accommodation area, which is significantly shifted towards the leading edge. That is very unfavorable at high subsonic speed at which long-range aircraft fly. The wings of BWB have a large span, which is unfavorable with regard to aerodynamic loading and manipulation at airports. The realistic gravity center position of the BWB aircraft cannot provide for the level of longitudinal stability and pitch maneuver required by the civil aviation rules. The future of this project is uncertain due to problems previously described.
My xe2x80x9cAirlifting-Surface-Divisionxe2x80x9d idea provides for the following advantages and applications:
a) Significant reduction of the induced drag on the tips of aerodynamic surfaces such as aircraft wings, helicopter, and windmill rotor blades with very slight increase in weight and parasitic drag.
b) Significant increase in the longitudinal stability and maneuverability of the aircraft based on the flying wing idea.
c) Providing for sufficient payload space with significant reduction of profile and wave drag due to ability to use thin laminar and supersonic airfoils at high subsonic and supersonic speeds for aircraft based on the flying wing idea.
Accordingly, besides the objects and advantages of the patents described above in my patent application, several objects of my xe2x80x9cAirlifting-Surface-Divisionxe2x80x9d idea are:
1. Significantly reduce induced drag on the tips of airlifting surfaces at high attack angles and thereby:
Provide for the high aerodynamic efficiency on the tips of airlifting surfaces
Minimize induced and interference drag by means of the new shape of airlifting surfaces tips.
2. Provide for required longitudinal stability, which is regulated by the civil aviation rules for the aircraft designed on the flying wing idea and, simultaneously, provide for a good pitch maneuver with relatively small stabilizing and maneuvering surfaces.
3. Provide for sufficient payload space for the aircraft designed on the flying wing idea and simultaneously minimize profile and waive drag at high subsonic or supersonic speeds.
4. Reduce wing span of large aircraft, especially aircraft designed on the flying wing idea, in order to improve maneuver on runways and provide for better access to the boarding gates at airports.
5. Reduce wing span of the classic-concept aircraft in order to minimize wing weight, which would result either in reduced fuel consumption or increased range or payload.
Still further objects and advantages will become apparent from a consideration of the ensuing description and drawings.
Although the description contains many specifics, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention.
Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.